SDM is broadly exploited in modern electronic systems, such as systems involving discrete-time digital signals sampled at a sampling frequency. Please refer to FIG. 1 illustrating a conventional spectrum arrangement for baseband signal(s) at a low-pass band. Because of many practical engineering considerations (e.g., matching an integer multiplication of a reference frequency), a sampling frequency fs of the signal(s) is usually set much higher than a Nyquist sampling frequency f_N of the signal(s). The spectrum therefore has a large vacancy, which extends from the low-pass band to an half of the sampling frequency (fs/2), not fully utilized.